每日一刷：验证回文数

输入: "A man, a plan, a canal: Panama"
输出: true

输入: "race a car"
输出: false

class Solution {
    public boolean isPalindrome(String s) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            if (Character.isLetterOrDigit(c)){
                sb.append(c);
            }
        }
        StringBuilder sbReverse = new StringBuilder(sb).reverse();
        return sb.toString().equalsIgnoreCase(sbReverse.toString());
    }
}

class Solution {
    public boolean isPalindrome(String s) {
        if (s.isEmpty()) {
            return true;
        }
        s = s.toLowerCase();

        for (int i = 0, j = s.length() - 1; i < j; i++, j--) {
            while (i < j && !Character.isLetterOrDigit(s.charAt(i))) {
                i++;
            }
            while (i < j && !Character.isLetterOrDigit(s.charAt(j))) {
                j--;
            }
            if (s.charAt(i) != s.charAt(j)) {
                return false;
            }
        }
        return true;
    }
}